The applicant previously described in U.S. Pat. No. 4,052,648 a power reduction system for induction motors in which the effective voltage input to the motor is varied directly as a function of load. In that patent it is noted that the power factor of such motors varies directly with loading, and that the power factor of a less than fully loaded motor might be maintained at an optimum level by reducing the effective voltage to the motor, the applicant earlier determined, and such is described in referenced patent, that power input to a motor may be automaticaly varied as a function of load by commanding that it operate at a selected power factor. The system described in this patent has been very successful and has been widely used with respect to single phase motors.
The applicant also tested the applicability of this system to three phase motors, testing being of motors wherein the "Y", or common reference power, terminal of the motor is brought out of the motor and available for control purposes. It worked well. In many three phase motors, however, it has been found that a common reference terminal is not available, that is, a common reference terminal through which current from all three phases flow. It further appears that simply sampling one of the three phases of the motor and deriving a power factor control signal from it is not satisfactory. Significant stability problems may be encountered.
It is an object of this invention to provide an improved three phase power factor controller particularly adaptable for employment with three phase motors wherein there is not available a terminal common to all phases, and wherein problems of motor instability are overcome.